Majority Spin Electrons Research Articles

High Electrical Conductance in Magnetic Emission Junction of Fe3GeTe2/ZnO/Ni Heterostructure via Selective Spin Emission through ZnO Ohmic Barrier.

The insulator is essential for magnetic tunneling junction (MTJ) that increases magnetoresistance (MR) by decoupling magnetization directions between two ferromagnets. However, wide bandgap tunnel barrier blocks the thermionic emission of electrons, significantly reducing electrical conductance through MTJ. Here, a magnetic emission junction (MEJ) is demonstrated for the first time using an Fe3GeTe2 (FGT)/ZnO/Ni heterostructure with very high electrical conductance. The conduction band of ZnO (electron affinity 4.6eV) aligns with Fermi levels (EF) of FGT (4.47eV) and Ni (4.58eV) ferromagnets and forms an Ohmic barrier, enabling free spin-electron emission through ZnO barrier and high electrical conductance. In contrast to the typical positive MR in MTJ by majority spin tunneling, negative MR is observed in FGT/ZnO/Ni MEJ. The minority spin electrons of Ni, with maximum states near the EF, are dominantly emitted to FGT over the ZnO barrier, while majority spin electrons of Ni, with maximum states below the EF, are blocked by it. In the FGT/FGT/ZnO/Ni heterostructure, the MR ratio is further increased by combining positive and negative MR at the MTJ (FGT/FGT) and MEJ (FGT/ZnO/Ni), respectively. As a result, FGT-MEJ exhibits 10-1000 orders higher conductance than other 2D-MTJs, while MR ratio remains similar to other 2D-MTJs.

First-Principles Calculations to Investigate Spin-Oriented Magnetic Phases, Band Gaps, Elastic, and Thermodynamic Calculations of VNi2O4

In this study, spin and U-Coulomb interactions-dependent electronic structure, pressure-dependent elastic, and thermodynamics studies of VNi2O4 were performed. The ferromagnetic (FM) phase was the most stable magnetic phase. The lattice parameter was calculated as 8.32 Å. The bulk modules were calculated as 197.695 GPa and 198.038 GPa, respectively, in the initial condition and the elastic calculations, so the initial and elastic conditions were compatible. VNi2O4 material had band gaps of 1.597 eV, 2.159 eV, 2.659 eV, 3.094 eV, 3.439 eV, 3.729 eV, and 3.978 eV in the majority spin electrons in the Generalized Gradient Approximation (GGA), and U-Coulomb interactions (U = 1, 2, 3, 4, 5 and 6 eV), respectively. Hence, it was a true half-metallic ferromagnetic. VNi2O4 was ductile at 0 GPa, with a Debye temperature of 586.596 K. The net magnetic moment of VNi2O4 was 6.00 µB/f.u. As a result, VNi2O4 provides all the necessary conditions for spintronic applications.

A Device Model for Achieving Sizable and Tunable Tunneling Magnetoresistance Using Two‐Dimensional Materials InX (X = O, Se) without Hetero‐Interface

AbstractThe two‐dimensional (2D) materials InX (X = O, Se), experimentally available thus far, can become a ferromagnetic half metal under hole doping, though the charge neutral states of them are nonmagnetic semiconductors. Based on such an electronic characteristic, a theoretical model of magnetic tunnel junction (MTJ) is proposed composed only of one of the 2D InX. In doing so, the two semi‐infinite pieces of 2D InX in the half‐metallic state is assumed as the opposite electrodes which are separated by a strip of the same material but in its nonmagnetic state. Owing to the 2D nature of InX, the half metal electrodes of the InX device induced by hole doping can be achieved by using split gating technique. The numerical simulations identify a proper hole doping concentration, at which 100% tunneling magnetoresistance (TMR) ratios can be realized, accompanying an appreciable conductance of majority spin electron under parallel magnetization configuration. Under a finite bias voltage, the TMR ratio remains high. Therefore, the proposed device model is an ideal candidate for future spintronics applications. It enables electrical control of TMR and circumvents the detriment of hetero‐interface disorder inevitable in conventional MTJs.

Indications of a ferromagnetic quantum critical point in textrm{SmN}_{1-delta }

We investigate the previously observed superconductivity in ferromagnetic SmN in the context of the breakdown of order between two magnetic phases. Nitrogen vacancy doped SmN_{1-delta } is a semiconductor which lies in the intermediary between ferromagnetic SmN and anti-ferromagnetic Sm. Optical data reported here corroborate the prediction that electrical transport is mediated by Sm 4f defect states, and electrical transport measurements characterise the metal-insulator transition over the doping range. Our measurements show that the superconducting state in nitrogen vacancy doped textrm{SmN}_{1-delta } is the most robust near the breakdown of magnetic order, and indicate the location of a quantum critical point. Furthermore we provide additional evidence that the superconducting state is formed from majority spin electrons and thus of unconventional S = 1 type.

Interplay of Magnetic Interaction and Electronic Structure in New Structure RE-12442 Type Hybrid Fe-Based Superconductors

We present detailed first-principles density functional theory-based studies on RbRE2Fe4As4O2 (RE = Sm, Tb, Dy, Ho) hybrid 12442-type iron-based superconducting compounds with particular emphasis on competing magnetic interactions and their effect on possible magneto-structural coupling and electronic structure. The stripe antiferromagnetic (sAFM) pattern across the xy plane emerges as the most favorable spin configuration for all the four compounds, with close competition among the different magnetic orders along the z-axis. The structural parameters, including arsenic heights, Fe-As-Fe angle, and other relevant factors that influence superconducting Tc and properties, closely match the experimental values in stripe antiferromagnetic arrangement of Fe spins. Geometry optimization with inclusion of explicit magnetic ordering predicts a spin–lattice coupling for all the four compounds, where a weak magneto–structural transition, a tetragonal-to-orthorhombic structural transition, takes place in the relaxed stripe antiferromagnetic spin configuration. Absence of any experimental evidence of such structural transition is possibly an indication of nematic transition in RE-12442 compounds. As a result of structural distortion, the lattice contracts (expands) along the direction with parallel (anti-parallel) alignment of Fe spins. Introduction of stripe antiferromagnetic order in Fe sub-lattice reconstructs the low-energy band structure, which results in significantly reduced number of bands crossing the Fermi level. Moreover, the dispersion of bands and their orbital characteristics also are severely modified in the stripe antiferromagnetic phase similar to BaFe2As2. Calculations of exchange parameters were performed for all the four compounds. Exchange coupling along the anti-parallel alignment of Fe spins J1a is larger than that for the parallel aligned spins J1b. A crossover between the super-exchange-driven in-plane next-nearest-neighbor exchange coupling J2 and in-plane exchange coupling J1a due to lanthanide substitution was found. A large super-exchange-driven next-nearest-neighbor exchange interaction is justified using the construction of 32 maximally localized Wannier functions, where the nearest-neighbor Fe-As hopping amplitudes were found to be larger than the nearest- and the next-nearest-neighbor Fe-Fe hopping amplitudes. We compare the hopping parameters in the stripe antiferromagnetic pattern with non-magnetic configuration, and increased hopping amplitude was found along the anti-parallel spin alignment with more majority-spin electrons in Fe dxz and dxy but not in Fe dyz. On the other hand, the hopping amplitudes are increased in stripe antiferromagnetic phase along the parallel spin alignment with more majority-spin electrons in only Fe dyz. This difference in hopping amplitudes in the stripe antiferromagnetic order enables more isotropic hopping.

Interplay between diffusion and magnon-drag thermopower in pure iron and dilute iron alloy nanowire networks

Results of measurements on the thermoelectric power of 45 nm diameter interconnected nanowire networks consisting of pure Fe, dilute FeCu and FeCr alloys and Fe/Cu multilayers are presented. The thermopower values of Fe nanowires are very close to those found in bulk materials, at all temperatures studied between 70 and 320 K. For pure Fe, the diffusion thermopower at room temperature, estimated to be around − 15 upmuV/K from our data, is largely supplanted by the estimated positive magnon-drag contribution, close to 30 upmuV/K. In dilute FeCu and FeCr alloys, the magnon-drag thermopower is found to decrease with increasing impurity concentration to about 10 upmuV/K at 10% impurity content. While the diffusion thermopower is almost unchanged in FeCu nanowire networks compared to pure Fe, it is strongly reduced in FeCr nanowires due to pronounced changes in the density of states of the majority spin electrons. Measurements performed on Fe(7 nm)/Cu(10 nm) multilayer nanowires indicate a dominant contribution of charge carrier diffusion to the thermopower, as previously found in other magnetic multilayers, and a cancellation of the magnon-drag effect. The magneto-resistance and magneto-Seebeck effects measured on Fe/Cu multilayer nanowires allow the estimation of the spin-dependent Seebeck coefficient in Fe, which is about − 7.6 upmuV/K at ambient temperature.

How deeply are core electrons perturbed when valence electrons are spin polarized? The case study of transition metal compounds.

The ferromagnetic and antiferromagnetic wave functions of the KMnF3 perovskite have been evaluated quantum-mechanically by using an all electron approach and, for comparison, pseudopotentials on the transition metal and the fluorine ions. It is shown that the different number of α and β electrons in the d shell of Mn perturbs the inner shells, with shifts between the α and β eigenvalues that can be as large as 6eV for the 3s level, and is far from negligible also for the 2s and 2p states. The valence electrons of F are polarized by the majority spin electrons of Mn, and in turn, spin polarize their 1s electrons. When a pseudopotential is used, such a spin polarization of the core functions of Mn and F can obviously not take place. The importance of such a spin polarization can be appreciated by comparing (i) the spin density at the Mn and F nuclear position, and then the Fermi contact constant, a crucial quantity for the hyperfine coupling, and (ii) the ferromagnetic-antiferromagnetic energy difference, when obtained with an all electron or a pseudopotential scheme, and exploring how the latter varies with pressure. This difference is as large as 50% of the all electron datum, and is mainly due to the rigid treatment of the F ion core. The effect of five different functionals on the core spin polarization is documented.

Semiconducting and magnetic properties within B(1-x)VxAs alloys at x = 0 and x = 0.25: A DFT + U study

The physical (mechanical (structural and elastic), electronic and magnetic) properties of the zincblende BAs parent compound and its B0.75V0.25As alloy are analyzed by employing the FP-L/APW + lo method as implemented in WIEN2k code, where the XC potential is approached by the GGA approximation. The analysis of the structural properties in B0.75V0.25As system indicates its stability in ferromagnetic state, where its computed equilibrium lattice parameters (the lattice constant (a0), the bulk modulus (B0), and the first-pressure derivative (B’) of the bulk modulus) are given. The elastic moduli (C11, C12, and C44) and anisotropy factor for the cubic system are determined for these compounds in the purpose to confirm their mechanical stability. The electronic structure of B0.75V0.25As shows its complete semiconducting feature. The electronic density of states indicates that the uncompleted 3d-V orbitals broaden on the width of the exchange splitting energies (Δx(d) and Δx(pd)); accordingly, the effective potential of minority-spin electrons are more attracted comparing to the majority-spin electrons. The magnetic properties unveil that the value of total magnetic moment of B0.75V0.25As alloy is found in an integer number. Thus, the hybridization between 3d-V and 4p-As states pushes to reduce the atomic magnetic moment of V element from its free space charge value and to produce local magnetic moments on B and As nonmagnetic sites. Moreover, the s-d exchange constant of conduction band (N0α) and the p-d exchange constant of valence band (N0β) are evaluated in the reason to know contributions during the exchange splitting process.

Spin-polarized cation monovacancies in wurtzite structure semiconductors: first-principles study

We study spin-polarized cation vacancies in wurtzite structure semiconductors (BeO, ZnO, ZnS, CdS, BN, AlN, GaN and GaP) by using first-principles calculations based on the density functional theory. We find that C3v geometries are the most stable and are spin-polarized. Two majority spin electrons occupying the defect E level lead to the magnetic moment of 2 μB in the case of II–VI semiconductors. On the contrary, in the case of III–V semiconductors, three majority spin electrons occupying the defect E and A1 levels induce the magnetic moment of 3 μB. The spin polarization of cation vacancies in oxides and nitrides are found to be stable compared with other cation vacancies in II–VI and III–V semiconductors, respectively. We clarify that the effect of the symmetry lowering from C3v to Cs is small and thus confirm that the spin polarized C3v geometries are the most stable.

Spin-polarized electron transmission through B-doped graphene nanoribbons with Fe functionalization: a first-principles study

In this study, we investigate the electron transport properties of a B-doped armchair graphene nanoribbon (AGNR) suspended between graphene electrodes based on first-principles calculations. Our calculations reveal that one of the electron transmission channels of a pristine AGNR junction is closed by the B-doping. We then proceed to explore the effect of the B-doping on the spin-polarized electron transport behavior of a Fe-functionalized AGNR junction. As a result, transmission channels for majority-spin electrons are closed and the spin polarization of the electron transmission is enhanced from 0.60 for the Fe-functionalized AGNR junction to 0.96 for the B- and Fe-codoped one. This observation implies that the codoped AGNR junction can be employed as a spin filter. In addition, we investigate the electronic nature of the transmission suppression caused by the B-doping. A detailed analysis of the scattering wave functions clarifies that a mode modulation of an incident wave arises in the B-doped AGNR part and the incident wave connects to an evanescent wave in the transmission-side electrode. For pristine and Fe-functionalized AGNR junctions, such a mode modulation is not observed and the incident wave connects to a propagating wave in the transmission-side electrode. Tuning of electron transport property by exploiting such a mode modulation is one of promising techniques for designing functionality of spintronics devices. We also discuss the general correspondence between the electron transmission spectrum and the density of states of a junction.

Tunable magnetoresistance and thermopower in interconnected NiCr and CoCr nanowire networks

Magnetoresistance and thermopower of crossed NiCr and CoCr nanowire networks have been measured as a function of temperature and chromium content in dilute alloys. At low temperatures, it is found that the impurity effect leads to negative anisotropic magnetoresistance, an observation that even persists until room temperature in diluted CoCr alloy nanowires. The addition of a small amount of Cr in nickel nanowires also abruptly reverses the sign of the thermopower from −20 μV/K for pure Ni up to +18 μV/K for the dilute alloys, implying the switching from n- to p-type conduction. These results are consistent with pronounced changes in the density of states for the majority spin electrons. The high room-temperature power factors of these magnetic nanowire networks (in the range of 1–10 mW/K2 m) provide interesting perspectives for designing n- and p-type legs for flexible spin thermoelectric devices.

Spin extraction from a semiconductor through a space-charge layer and critical current density

Simulations are carried out to study the extraction of spin-polarized electrons at a nonmagnetic semiconductor/ferromagnetic semiconductor junction in the presence of a space-charge layer. A spin blockade which limits the current flow in the structure is developed at such a junction when the outflow of majority spin electrons creates a cloud of minority spin electrons in the nonmagnetic semiconductor region near the junction. The presence of charge accumulated and depleted layers at the junction is observed to significantly modify the critical current density compared to that at charge neutral condition. The physical reason is investigated.

Ultrahigh spin transport performance in Ti2CoGe based magnetic tunnel junction

Abstract The magnetic tunnel junction (MTJ) device employing inverse Heusler alloy as electrode has great potential for application in spintronics. In this work, we performed first-principles calculations combined with nonequilibrium Green’s function to study the spin transport properties of Heusler alloy based Ti2CoGe/MgO/Ti2CoGe MTJ with various atomic terminations. The calculated interface free energy (γ) indicates that TiCo-terminated interface could be easily formed than TiGe-terminated interface, and the interface containing pure Ge atom is the most favorable owing to its lowest γ. Spin transport calculations reveal that tunnel magnetoresistance (TMR) ratio in MTJ with natural TiCo terminated interface (1.58 × 104) is higher than that with natural TiGe-terminated interface (2.99 × 103). When the interfacial Ti atom in TiCo-terminated interface is substituted by Co atom, the transport ability of majority spin electrons is promoted. Moreover, our calculation indicates that the MTJ with the interface containing pure Co atom possesses the largest TMR ratio of 9.51 × 105. Our work paves the way for the fabrication of Ti2CoGe based MTJ for practical application in spintronics.

Highly conductive and complete spin filtering of nickel atomic contacts in a nitrogen atmosphere

Generating efficient and highly spin-polarized currents through nanoscale junctions is essential in the field of nanoelectronics and spintronics. In this paper, using $ab$ $initio$ electron transport calculations, we predict highly conductive and perfect spin filtering of nickel atomic contacts in a nitrogen environment, where a single N$_2$ molecule sits in parallel (energetically most favorable) between two nickel electrodes. Such a particular performance is due to the wave function orthogonality between majority spin $s$-like states of ferromagnetic electrodes and the lowest unoccupied molecular orbital of the N$_2$ molecule, and thus, majority spin electrons are completely blocked at the interface. For the minority spin, on the contrary, two almost saturated conducting channels were formed due to the effective coupling between $d_{zx,zy}$ of the Ni atom and $p_{x,y}$ of the N atom, resulting in large conductance of about 1$G_0$ ($=2e^2/h$). As a consequence, a single N$_2$ molecule acts as a highly conductive and half-metallic conductor. On the other hand, the CO and NO incorporated molecular junctions exhibit rather low conductance with a partially spin-polarized current.

Computational investigation of CrFeZ [Z = Si, Sn and Ge] half-Heusler compounds ferromagnets

The structural, electronic, magnetic and mechanical properties of the ternary CrFeZ half-Heusler compounds (with Z = Si, Sn, Ge) have been determined ab initio using a full-potential linearized muffin tin orbital approach. Equilibrium properties such as lattice constant, bulk modulus and its pressure derivative are calculated. Spin-orbit interaction effect in the electronic structure and Fermi levels are revealed. The majority-spin electrons are found to be metallic in nature, while the minority-spin electrons are found to be semiconducting electronic band-structure. It is shown that calculated band structures, density of states, magnetic moments, and elastic constants of these alloys agree well with available with theoretical and experimental data. In addition, the investigated compounds are found to be mechanical stable. Our findings predict new properties, as-yet unreported elastic parameters in the C1b structure, and thus may be realized under ideal experimental circumstances, which make them potential candidates for future spintronic applications.

Tuning the electronic and magnetic property of semihydrogenated graphene and monolayer boron nitride heterostructure

The structural stability, electronic and magnetic properties of semihydrogenated graphene and monolayer boron nitride (H-Gra@BN) composite system are studied by the first principles calculation. First, for the six possible stacked configurations of H-Gra@BN in three kinds of magnetic coupling manners, including the nonmagnetic, ferromagnetic and antiferromagnetic, the geometry optimization structures are calculated. The formation energies (Ef) are -28, -37, -40, -35, -28, and -34 meV/atom for AA-B, AA-N, AB-B, AB-B-H, AB-N and AB-N-H configurations of H-Gra@BN, respectively. The details of the six H-Gra@BN configurations are presented. The results show that the AB-B configuration of H-Gra@BN system is most stable with the largest formation energy in the six configurations. Its thickness is the smallest in all six configurations. The formation energies of all configurations are very close to each other and show that the combination of the interlayer between layers is very weak, The interaction between H-Gra and monolayer BN is van der Waals binding. Second, the band structure, total density of states (TDOS), partial density of states and polarization charge density of the most stable H-Gra@BN system are systematically analyzed. This material is ferromagnetic semiconductor. The band gaps for majority and minority spin electrons are 3.097 eV and 1.798 eV, respectively. Each physical cell has an about 1 μB magnetic moment, which is mainly derived from the contribution of the unhydrogenated C2 atom. Furthermore, while the pressure is applied along the z direction, we analyze the TDOS and band structure of H-Gra@BN system, and find that when the z axis strain is more than -10.48% (Δh=-0.45 Å), the valence band maximum of minority spin moves down. The conduction band minimum of minority spin moves from the high symmetry Γ position into a position between Γ and K. The electronic properties of the most stable H-Gra@BN system change from magnetic semiconductor into half metal. When the strain is increased by more than -11.65% (Δh=-0.5 Å), the most stable H-Gra@BN changes into a nonmagnetic metal. To analyze the effect caused by different strains, we analyze the difference in three-dimensional charge density, and find that with the decrease of the layer spacing, the interlayer interaction gradually increases and shows the obvious covalent bond characteristics. This paper predicts a new type of two-dimensional material of which the electronic and magnetic properties can be easily tuned by pressure, and it is expected to be used in nano-devices and serve as an intelligent building material.

Charge transfer and magnetization of a MoS2 monolayer at the Co(0001)/MoS2 interface

The Co/MoS2 system may constitute a fundamental building block for future spintronic devices based on a single MoS2 transition metal dichalcogenide monolayer. Here, the hcp Co(0001)/MoS2 interface electronic structure as well as magnetic properties are investigated by first principles calculations based on the density functional theory. The charge transfer due to covalent bonding between S and Co atoms at the interface has been calculated for the lowest energy configuration obtained after optimization of the atomic coordinates. This charge transfer is different for majority and minority spin electrons, which induces a magnetization of the MoS2 layer bellow the Cobalt contact. The connection between the charge transfers at the interface and the modification of the magnetic properties is discussed.

Effects of on-site Coulomb interaction (U) on the structural and electronic properties of half-metallic ferromagnetic orthorhombic Pr0.75Na0.25MnO3 manganite: a LDA + U calculation and experimental study

In this study, the structural, electronic, and half-metallic properties of monovalent-doped Pr0.75Na0.25MnO3 manganite were investigated via density functional theory (DFT) with local density approximation (LDA) and local density approximation plus Hubbard U parameter (LDA + U). The effect of on-site Coulomb interaction on the structural and electronic properties of REMnO3 (RE = Pr, Nd, La) manganites, which consist of strongly interacting RE 4f and transition metal Mn 3d electrons, were studied. The strong Coulomb repulsion between electrons was corrected using the Hubbard parameter U, which ranged from 2 to 6 eV for Mn 3d and was set at 6 eV for Pr 4f. The calculated results showed that the structural parameters, charge and spin densities, and the total and partial densities of states of Pr0.75Na0.25MnO3 manganite were sensitive to the change in U value. The crystal structure and energy band gap of Pr0.75Na0.25MnO3 manganite were calculated using LDA + U with the optimized U value of 2 eV for Mn 3d. The results calculated with a U value of 2 eV showed better agreement with the experimental data compared with those calculated with U values of 4 eV and 6 eV. Interestingly, the LDA + U-based calculations revealed that Pr0.75Na0.25MnO3 manganite has a half-metallic ferromagnetic character. The majority-spin electrons exhibited metallic behavior, whereas the minority-spin electrons exhibited insulating gaps of 2.50, 3.28, and 4.20 eV for the U values of 2, 4, and 6 eV, respectively. The calculated band structure and density of states indicated that the on-site Coulomb repulsion U significantly influenced the hybridization of O 2p with Mn 3d orbitals at the valence and conduction bands.

Electronic and Magnetic Properties of SnFe2O4 Spinel Ferrites

In this work, the electronic and magnetic properties of SnFe2O4 spinel ferrite with various cation distributions (mixed, normal and inverse spinel phases) were studied. The calculations were performed by Korringa–Kohn–Rostoker (KKR)-coherent potential approximation (CPA) method with generalized gradient (GGA) approximation for the exchange and correlation functional. Our spin-polarized calculations give a half-metallic character for SnFe2O4 inverse spinel phase and near half-metallic character for SnFe2O4 mixed spinel phase and metal character for SnFe2O4 normal spinel phase. These results, which contribute to understanding the effect of the cation distribution in SnFe2O4 ferrite with spinel structure in the existence of gap states occupied by majority and minority spin electrons, the spin magnetic moments, the magnetization and the half-metallic behaviour.

Spin-Polarized Positron Annihilation Study on Some Ferromagnets

We briefly review the spin-polarized positron annihilation experiments on some ferromagnets (Fe, Co, Ni, Gd, Co2MnSi, Co2MnAl and NiMnSb) using positron beams generated with 68Ge-68Ga sources. The differential DBAR spectra between majority and minority spin electrons are well interpreted by the first principles band structure calculation. This further provides information about the half-metallicity of the Heusler alloys. The surfaces of Fe, Co and Ni are more negatively spin-polarized, that is, there are more majority than minority spin electrons. To explain the observed spin polarization quantitatively, detailed theoretical calculations and further experiments are required.